U.S. patent application number 11/525012 was filed with the patent office on 2007-03-29 for nitride semiconductor device.
This patent application is currently assigned to SAMSUNG ELECTRO-MECHANICS CO., LTD.. Invention is credited to Jae Woong Han, Soo Han Kim, Seong Suk Lee, Jong Hak Won.
Application Number | 20070069234 11/525012 |
Document ID | / |
Family ID | 37892776 |
Filed Date | 2007-03-29 |
United States Patent
Application |
20070069234 |
Kind Code |
A1 |
Won; Jong Hak ; et
al. |
March 29, 2007 |
Nitride semiconductor device
Abstract
A nitride semiconductor device is provided. In the device, first
and second conductivity type nitride layers are formed. An active
layer is formed between the first and second conductivity type
nitride layers. The active layer includes at least one quantum
barrier layer and at least one quantum well layer. Also, a current
spreading layer is interposed between the first conductivity type
nitride layer and the active layer. The current spreading layer has
an In content greater than the quantum well layer of the active
layer.
Inventors: |
Won; Jong Hak; (Suwon,
KR) ; Kim; Soo Han; (Sungnam, KR) ; Han; Jae
Woong; (Sungnam, KR) ; Lee; Seong Suk; (Suwon,
KR) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
SAMSUNG ELECTRO-MECHANICS CO.,
LTD.
|
Family ID: |
37892776 |
Appl. No.: |
11/525012 |
Filed: |
September 22, 2006 |
Current U.S.
Class: |
257/103 ;
257/E33.008 |
Current CPC
Class: |
H01S 5/2009 20130101;
H01L 33/06 20130101; B82Y 20/00 20130101; H01S 5/20 20130101; H01S
5/34333 20130101; H01S 2301/173 20130101; H01L 33/32 20130101 |
Class at
Publication: |
257/103 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 23, 2005 |
KR |
10-2005-0088772 |
Claims
1. A nitride semiconductor device comprising: first and second
conductivity type nitride layers; an active layer formed between
the first and second conductivity type nitride layers, the active
layer including at least one quantum barrier layer and at least one
quantum well layer; and a current spreading layer interposed
between the first conductivity type nitride layer and the active
layer, the current spreading layer having an In content greater
than the quantum well layer of the active layer.
2. The nitride semiconductor device according to claim 1, wherein
the current spreading layer has an In content that is at least 5
mol % greater than the quantum well layer.
3. The nitride semiconductor device according to claim 1, wherein
the active layer includes a quantum barrier layer having a
composition expressed by In.sub.x1Ga.sub.1-x1N, where
0.ltoreq.x.sub.1<1 and a quantum well layer having a composition
expressed by In.sub.x2Ga.sub.1-x2N, where x.sub.1<x.sub.2<1,
and wherein the current spreading layer has a composition expressed
by In.sub.yGa.sub.1-yN, where x.sub.2<y.ltoreq.1.
4. The nitride semiconductor device according to claim 3, wherein
the current spreading layer has an In content that is at least 5
mol % greater than the quantum well layer.
5. The nitride semiconductor device according to claim 3, wherein
the current spreading layer includes at least one pair of first and
second layers stacked alternately and having a different
composition from each other, and wherein the first layer has a
composition expressed by In.sub.yGa.sub.1-yN and the second layer
is a GaN layer.
6. The nitride semiconductor device according to claim 5, wherein
the current spreading layer is formed such that the first and
second layers alternate with each other by two to twelve plies.
7. The nitride semiconductor device according to claim 5, wherein
the first layer has a thickness ranging from 10 .ANG. to 100
.ANG..
8. The nitride semiconductor device according to claim 5, wherein
the second layer has a thickness ranging from 100 .ANG. to 250
.ANG..
9. The nitride semiconductor device according to claim 1, wherein
the first conductivity type nitride layer comprises an n-type
nitride layer and the second conductivity type nitride layer
comprises a p-type nitride layer.
Description
CLAIM OF PRIORITY
[0001] This application claims the benefit of Korean Patent
Application No. 2005-88772 filed on Sep. 23, 2005 in the Korean
Intellectual Property Office, the disclosure of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a nitride-based nitride
semiconductor device, more particularly which is improved in
internal quantum efficiency, operating voltage and inverse voltage
properties.
[0004] 2. Description of the Related Art
[0005] In general, a nitride semiconductor layer is broadly applied
to a green or blue light emitting diode (LED) used as a light
source in full-color displays, image scanners, various signal
systems and optical telecommunication devices, and a laser diode
(LD).
[0006] Efficiency of the nitride semiconductor device is determined
by internal quantum efficiency. i.e., recombination probability of
electrons and holes in an active layer. In a major effort to boost
the internal quantum efficiency, the active layer has been
structurally improved and effective mass of carriers has been
increased. Meanwhile, current crowding is mainly responsible for
decline in efficiency of the nitride semiconductor device since
carriers are not uniformly injected into the active layer.
[0007] Especially, current crowding is aggravated by inevitable
arrangement of electrodes in the nitride semiconductor device. That
is, when the nitride semiconductor device includes an insulating
substrate such as a sapphire substrate, two electrodes are not
disposed on opposed faces, but formed to face the same direction by
mesa-etching an epitaxial layer. This is a planar nitride
semiconductor light emitting device.
[0008] Such a planar nitride semiconductor device is disadvantaged
in some aspects over a vertical light emitting device which has two
electrodes formed on opposed faces of a light emitting structure.
That is, in the planar nitride semiconductor device, current fails
to travel uniformly across an overall light emitting area, thereby
decreasing an effective light emitting area and also light emitting
efficiency per light emitting area.
[0009] Therefore, in the art, to manufacture a high-efficiency
nitride semiconductor device, there has arisen a need for a novel
nitride semiconductor device which can spread current uniformly
across the overall active layer to enhance light emitting
efficiency.
SUMMARY OF THE INVENTION
[0010] The present invention has been made to solve the foregoing
problems of the prior art and therefore an object according to
certain embodiments of the present invention is to provide a novel
nitride semiconductor device which adopts, between an active layer
and a clad layer, a layer having a relatively high In content and
also high carrier (especially electrons) mobility, thereby further
allowing current to flow laterally and be spread more
uniformly.
[0011] According to an aspect of the invention for realizing the
object, there is provided a nitride semiconductor device including
first and second conductivity type nitride layers; an active layer
formed between the first and second conductivity type nitride
layers, the active layer including at least one quantum barrier
layer and at least one quantum well layer; and a current spreading
layer interposed between the first conductivity type nitride layer
and the active layer, the current spreading layer having an
In-content greater than the quantum well layer of the active
layer.
[0012] The active layer includes a quantum barrier layer having a
composition expressed by In.sub.x1Ga.sub.1-x1N, where
0.ltoreq.x.sub.1<1 and a quantum well layer having a composition
expressed by In.sub.x2Ga.sub.1-x2N, where x.sub.1<x.sub.2<1,
and the current spreading layer has a composition expressed by
In.sub.yGa.sub.1-yN, where x.sub.2<y.ltoreq.1.
[0013] To sufficiently ensure current to flow laterally, high
carrier mobility should be guaranteed. Preferably, the current
spreading layer has an In content that is at least 5 mol % greater
than the quantum well layer. Consequently the current spreading
layer exhibits higher carrier mobility than the active layer.
[0014] In a preferable embodiment of the invention, the current
spreading layer includes at least one pair of first and second
layers stacked alternately and having a different composition from
each other, and the first layer has a composition expressed by
In.sub.yGa.sub.1-yN and the second layer is a GaN layer. More
preferably, the current spreading layer is formed such that the
first and second layers alternate with each other by two to twelve
plies. The at least two plies assure a sufficient effect and the
plies in excess of twelve increase thickness of an overall layer
and accordingly resistance, thereby degrading light emitting
efficiency.
[0015] Preferably, the first layer has a thickness ranging from 10
.ANG. to 100 .ANG. and the second layer has a thickness ranging
from 100 .ANG. to 250 .ANG.. But the invention is not limited
thereto.
[0016] The current spreading layer with a high In content according
to the invention is interposed between the n-type nitride layer and
the active layer, i.e. an area which experiences severe current
crowding, thereby further enhancing light emitting efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The above and other objects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0018] FIG. 1 is a side sectional view illustrating a nitride
semiconductor device according to an embodiment of the
invention;
[0019] FIG. 2a is a side sectional view illustrating a nitride
semiconductor device according to a preferred embodiment of the
invention;
[0020] FIG. 2b is an energy band diagram illustrating the nitride
semiconductor device shown in FIG. 2a;
[0021] FIG. 3 is a schematic view for explaining effects of a
current spreading layer according to the embodiment of FIG. 2a;
and
[0022] FIG. 4 is a graph illustrating EL spectrums to compare light
emitting efficiency between a conventional light emitting device
and a light emitting device according to an embodiment of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] Preferred embodiments of the present invention will now be
described in detail with reference to the accompanying
drawings.
[0024] FIG. 1 is a side sectional view illustrating a nitride
semiconductor device 10 according to an embodiment of the
invention.
[0025] Referring to FIG. 1, the nitride semiconductor device 10
includes a buffer layer 12, an n-type nitride semiconductor device
14, an active layer 16 and a p-type nitride semiconductor layer 17
stacked sequentially on a substrate 11.
[0026] The substrate 11 may be made of e.g., sapphire or SiC, which
is a different material from the semiconductor device 10, or GaN,
which is the same material as the semiconductor device 10. The
active layer 16 may be of a multiple quantum well structure
including a quantum well structure 16a and a quantum barrier layer
16b. Also, the p-type nitride semiconductor layer 17 may include a
P-type AlGaN layer 17a for preventing electron overflowing and a
P-type GaN layer 17b for providing a contact area.
[0027] The nitride semiconductor device 10 according to this
embodiment includes an InGaN current spreading layer 15 formed
between the n-type nitride semiconductor layer 14 and the active
layer 16. The current spreading layer 15 has In content greater
than the quantum well layer 16a. In general, purportedly a nitride
layer with a great In content assures higher carrier mobility than
a nitride layer of different compositions.
[0028] For example, electron mobility (about 3200 cm'/Vs) in an InN
layer is much higher than that (about 300 cm'/Vs, 1000 to 1350
cm'/Vs, respectively) in an AlN layer and a GaN layer.
[0029] Consequently, electrons are guaranteed to be highly mobile
in the InGaN current spreading layer 15, thereby further ensuring
current to flow laterally and thus be spread more uniformly. To
sufficiently improve such lateral flow of current, preferably the
current spreading layer 15 has an In content that is at least 5 mol
% greater than the quantum well layer. This produces sufficient
current spreading effect.
[0030] Preferably, in a case where the quantum barrier layer 16b is
a nitride layer having a composition expressed by
In.sub.x1Ga.sub.1-x1N, where 0.ltoreq.x.sub.1<1 and the quantum
well layer 16a has a composition expressed by
In.sub.x2Ga.sub.1-x2N, where x.sub.1<x.sub.2<1, the current
spreading layer 15 has a composition expressed by
In.sub.yGa.sub.1-yN, where x.sub.2<y.ltoreq.1.
[0031] FIG. 2a is a side sectional view illustrating a nitride
semiconductor device according to a preferred embodiment of the
invention.
[0032] The nitride semiconductor device 20, in a similar manner to
FIG. 1, includes a buffer layer 22, an n-type GaN layer 24, an
active layer 26 of a multiple quantum well structure and a p-type
AlGaN/GaN layer 27a and 27b stacked sequentially on a substrate
21.
[0033] The nitride semiconductor device 20 according to this
embodiment includes a current spreading layer 25 interposed between
the n-type nitride semiconductor layer 24 and the active layer
26.
[0034] The current spreading layer 25 according to this embodiment
may include a first layer 25a made of InGaN and a second layer 25b
made of GaN, which are preferably stacked alternately more than
once.
[0035] Conventionally, the n-type nitride layer and active layer
have often employed an InGaN/GaN superlattice structure to improve
crystallinity. Here, the InGaN layer is required to have an In
content lower than the quantum well layer in view of lattice match.
On the other hand, the first layer 25a according to the invention
necessarily has an In content greater than the quantum well layer
26a. As described above, a greater In content increases electron
mobility, thereby ensuring current to be spread more uniformly.
Preferably, the first layer 25a has an In content of at least 15
mol %.
[0036] The first layer 25a corresponds to the InGaN current
spreading layer 15 shown in FIG. 1. The first layer 25a allows high
electron mobility, further ensuring current to flow laterally and
accordingly be spread uniformly. In a case where the quantum
barrier layer 26b is a nitride layer having a composition expressed
by In.sub.x1Ga.sub.1-x1N, where 0.ltoreq.x.sub.1<1, and the
quantum well layer 26b has a composition expressed by
In.sub.x2Ga.sub.1-x2N, where x.sub.1<x.sub.2<1, the first
layer 25a of the current spreading layer 25 may have a composition
expressed by In.sub.yGa.sub.1-yN, where x.sub.2<y.ltoreq.1. FIG.
2b is an energy band diagram illustrating construction of such a
current spreading layer 25.
[0037] Moreover, this embodiment of the invention improves current
spreading effect further than the embodiment in which the current
spreading layer 25 is structured as a single layer (see FIG. 1).
That is, in this embodiment, the InGaN first layer 25a is disposed
alternately with the GaN second layer 25b more than once, thereby
further enhancing current spreading effect. The effect is more
readily understood by a schematic current flow in the current
spreading layer 25 shown in FIG. 3.
[0038] Even if current injected into the current spreading layer 25
is crowded in a certain area (e.g. a central area) as indicated
with arrows (see FIG. 3), current is spread laterally in the first
layer 25a having higher electron mobility than the GaN second layer
25b. Moreover, the current spreading effect is gradually increased
while the current enters other first layers 25a through other
second layers 25b. In this fashion, according to the invention, the
InGaN first layer 25a and the GaN second layer 25b are stacked
alternately, thereby allowing current to be spread laterally and
more uniformly. This significantly boosts light emitting
efficiency.
[0039] Preferably, the first and second layers 25a and 25b of the
current spreading layer 25 and 25b are stacked by two to twelve
plies. The at least two plies ensure a sufficient effect and the
plies in excess of twelve increase thickness of an overall layer 25
and accordingly resistance. This degrades light emitting
efficiency.
[0040] Preferably, the first layer 25a has a thickness ranging from
10 .ANG. to 100 .ANG. but is not limited thereto. The thickness
less than 10 .ANG. hardly yields current spreading effect and the
thickness in excess of 100 .ANG. may potentially deteriorate
crystallinity due to lattice mismatch. Also, preferably, the second
layer 25b has a thickness ranging from 100 .ANG. to 250 .ANG..
Here, the thickness less than 100 .ANG. is unlikely to produce a
desired effect owing to a tunneling effect. The thickness in excess
of 250 .ANG. increases thickness of the overall layer excessively,
accordingly raising series resistance.
[0041] Operation and effects of the invention will be explained
hereunder in more detail by way of a detailed example according to
the invention.
EXAMPLE
[0042] To improve light emitting efficiency of a nitride
semiconductor device according to the invention, the nitride light
emitting device (LED) was manufactured via Metal Organic Chemical
Vapor Deposition (MOCVD) in a reactor of NH.sub.3 atmosphere.
[0043] First, a GaN low-temperature nucleation growth layer was
formed on a sapphire substrate. Then, an n-type GaN layer was grown
to a thickness of 3 .mu.m and at a doping concentration of
2.times.10.sup.18/cm' via Si.
[0044] Subsequently, as suggested by the invention, a first layer
made of In.sub.0.28Ga.sub.0.72N and a second layer made of GaN were
stacked alternately with each other four times, thereby forming a
current spreading layer. Here, the first layer had a thickness
ranging from 30 .ANG. to 40 .ANG. and the second layer had a
thickness of 130 .ANG. to 140 .ANG..
[0045] Next, an active layer of a single quantum well structure was
formed on the current spreading layer. The active layer was
comprised of an In.sub.0.18Ga.sub.0.82N quantum well layer and a
GaN quantum well layer to obtain a light of about 450 nm
wavelength.
[0046] On the active layer were grown an AlGaN layer to a thickness
of 30 nm and at a doping concentration of about 2.about.5
.times.10.sup.17/cm', and a p-type GaN layer to a thickness of
about 120 nm.
Comparative Example
[0047] A nitride light emitting device was manufactured in a manner
equal to the aforesaid Example. The Comparative Example adopted a
superlattice structure which was conventionally employed to enhance
crystallinity, in substitute for an electron blocking layer. That
is, a first layer of In.sub.0.18Ga.sub.0.82N having an In content
equal to a quantum well layer and a second layer of GaN were
stacked alternately with each other four times. Here, the first
layer had a thickness of 30 .ANG. to 40 .ANG. and the second layer
had a thickness of 130 .ANG. to 140 .ANG. in the same manner as in
the Example.
[0048] An electro-luminescence (EL) spectrum was measured for the
two nitride light emitting devices obtained. FIG. 4 illustrates the
results.
[0049] As shown in FIG. 4, the conventional light emitting device b
employing a simple superlattice structure exhibited an EL spectrum
of 1200 at a wavelength band of 450 nm. On the other hand, the
light emitting device a manufactured according to the embodiment
demonstrated an EL spectrum of 1400. These results have confirmed
that the light emitting device employing the current spreading
layer according to the invention is improved in light emitting
efficiency by about 17% compared to the conventional light emitting
device which merely shows better crystallinity. The current
spreading layer according to the invention may be comprised of a
single layer or a structure where the current spreading layer and a
GaN layer are stacked alternately with each other. Furthermore, the
current spreading layer of the invention has a greater In content
than the quantum well layer, thereby enhancing electron mobility
and hole mobility therein. Also, the current spreading layer, when
interposed between the p-type nitride layer and the active layer,
yields similar effects.
[0050] In addition, a light emitting device structure having an
active layer of a multiple quantum well structure is depicted in
the accompanying drawings of the invention. But the light emitting
device may adopt an active layer of a single quantum well structure
as in the Example. Further, the invention is beneficially
applicable to other nitride semiconductor devices such as a light
emitting diode (LED) and a laser diode (LD).
[0051] As set forth above, according to preferred embodiments of
the invention, a layer having a relatively high In content is
sandwiched between an active layer and a clad layer to guarantee
high carrier (especially electron) mobility. This further ensures
current to flow laterally and thus be spread more uniformly across
an overall area, thereby producing a high efficiency nitride
semiconductor device.
[0052] While the present invention has been shown and described in
connection with the preferred embodiments, it will be apparent to
those skilled in the art that modifications and variations can be
made without departing from the spirit and scope of the invention
as defined by the appended claims.
* * * * *